Database Management: Delete old conversations table

[kartpop]

Is your feature request related to a problem?
The old OpenAI conversations/assistant table has not seen any activity since June 2026, and it is no longer serving its purpose. This leads to confusion and unnecessary clutter in our system.

Describe the solution you'd like

• Completely delete the old OpenAI conversations/assistant table (with migration).
• Mark the corresponding API as deprecated and update documentation accordingly.

Original issue

Context

The old OpenAI conversations/assistant table is effectively dead — last real activity was June 2026.

Decision

• Delete the old OpenAI conversations/assistant table entirely.
• Mark the corresponding API as deprecated.

Acceptance criteria

• The dead table is removed (with migration).
• The corresponding API is documented/marked as deprecated.